Quantum Wells Of Different Widths Research Articles

Проявление обменного взаимодействия электронов, разделенных потенциальным барьером в двойных квантовых ямах, в эффекте Керра

In tunnel-coupled quantum wells of different widths, it was found that the spin dynamics arising from resonant pulsed optical pumping of an exciton in the narrow well includes the dynamics of electron spin polarization in the wide well, although the electron level in the wide well is 55 meV lower than the electron level in the narrow well. An analysis of the obtained results showed that the observed effect is caused by the exchange interaction of spin-polarized electrons in the wide well with exciton states in the narrow well.

Manifestation of the exchange interaction of electrons separated by a potential barrier in double quantum wells in the Kerr effect

In tunnel-coupled quantum wells of different widths, it was found that the spin dynamics arising from resonant pulsed optical pumping of an exciton in the narrow well includes the dynamics of electron spin polarization in the wide well, although the electron level in the wide well is 55 meV lower than the electron level in the narrow well. An analysis of the obtained results showed that the observed effect is caused by the exchange interaction of spin-polarized electrons in the wide well with exciton states in the narrow well. Keywords: semiconductor nanostructures, exchange interaction, Kerr effect.

Heavy-hole–light-hole exciton system in GaAs/AlGaAs quantum wells

Light-hole (lh) excitons (Xlhs) in quantum wells (QWs) are hardly studied compared with heavy-hole (hh) excitons (Xhhs), mainly due to the difficulties of their experimental observation. In this paper, a comprehensive study of both types of excitons in high-quality GaAs/AlGaAs QWs of different widths is performed. We focus on the energy positions of exciton resonances and the exciton-light interaction. The effect of mixing lhs and hhs in GaAs-based structures on these exciton characteristics is investigated. The corrections to the exciton energy due to mixing are only a fraction of millielectronvolts for wide QWs, in which the energy levels of Xhhs and Xlhs are close to each other. It is also experimentally found that the oscillator strength of Xlhs is $\ensuremath{\sim}2.5$ times less than that of Xhhs. This value noticeably deviates from the 3:1 oscillator strength ratio known for optical transitions between free electron and hole states. This deviation originates from the different squeezing of the Xlh and Xhh wave functions due to distinct values of the effective masses of the Xlh and Xhh in the heterostructure. Mixing of hh and lh valence bands is not so important.

Comparative Analysis of Giant Optical Fluctuations in GaAs Quantum Wells of Different Widths

Comparative Analysis of Giant Optical Fluctuations in GaAs Quantum Wells of Different Widths

Luminescence and carrier concentration in Sb-containing narrow bandgap quantum wells under optical excitation

We have experimentally studied the photoluminescence from InGaAsSb/AlGaAsSb quantum wells of different width under different levels of interband optical pumping. The dependences of photoluminescence intensity at spectral maxima on carrier concentration were theoretically calculated. This dependence was received using the rate equation taking into account the role of radiative and nonradiative recombination. Comparison of the theoretical and experimental results allowed to determine the nonequilibrium carrier concentration in quantum wells at certain experimental conditions.

Photoluminescence in InGaAsSb/AlGaAsSb quantum wells: impact of nonradiative recombination

The photoluminescence in nanostructures with InGaAsSb/AlGaAsSb quantum wells of different widths is investigated experimentally under different pumping levels and theoretically. The experimentally determined dependencies of photoluminescence intensity on the optical pumping level are compared with the calculated dependences of photoluminescence intensity on the nonequilibrium carrier concentration. The presence of resonant nonradiative Auger recombination in one of the investigated samples is proved by the comparison between experiment and calculations.

Interference effects in silicon-germanium heterostructures with quantum wells of different widths

Weak localization effects and the interactions of charge carriers are studied in two Si0.7Ge0.3/Si0.2Ge0.8/Si0.7Ge0.3 p-type heterostructures, where one or two quantum levels are filled, respectively. A weak localization effect for two-dimensional charge carriers is found to occur in weak magnetic fields when the spin-orbital and inelastic scattering times are close, which is indicative of splitting of the spin states under the influence of a perturbing potential related to the formation of a two-dimensional potential well (Rashba mechanism). In higher magnetic fields when one quantum level is occupied, interaction effects appear that are caused by Coulomb interactions with a scatterer. When the two quantum levels are occupied, the dominant mechanism is scattering on Friedel oscillations of the charge carrier density induced by an impurity electric field. In all regions, the quantum corrections are in good agreement with modern theoretical predictions.

Optical properties of ZnMnTe/ZnMgTe quantum-well nanostructures

The luminescence, luminescence excitation, reflection, and Raman scattering spectra of planar ZnMnTe/ZnMgTe nanostructures with ZnMnTe quantum wells of different widths and elemental composition have been studied. Based on the optical measurements, the energy levels of the nanostructures have been found. It has been shown that the intracenter luminescence excitation spectrum of manganese ions contains a detailed information on the electronic spectrum of the nanostructures. The factors affecting the band widths of exciton luminescence of the quantum wells has been considered. The Raman scattering spectra depends considerably on the parameters of the nanostructure and the relation between the energies of incident photons and intrinsic resonances of the nanostructure.

Quantum oscillations of photocurrents in HgTe quantum wells with Dirac and parabolic dispersions

We report on the observation of magneto-oscillations of terahertz radiation induced photocurrent in HgTe/HgCdTe quantum wells of different widths, which are characterized by a Dirac-like, inverted, and normal parabolic band structure. The photocurrent data are accompanied by measurements of photoresistance (photoconductivity), radiation transmission, as well as magnetotransport. We develop a microscopic model of a cyclotron-resonance assisted photogalvanic effect, which describes main experimental findings. We demonstrate that the quantum oscillations of the photocurrent are caused by the crossing of Fermi level by Landau levels resulting in the oscillations of spin polarization and electron mobilities in spin subbands. Theory explains a photocurrent direction reversal with the variation of magnetic field observed in experiment. We describe the photoconductivity oscillations related with the thermal suppression of the Shubnikov--de Haas effect.

Intra-center excitation dynamics in Cd0.6Mn0.4Te/Cd0.5Mg0.5Te superlattices and Cd0.5Mn0.5Te thin film

Intra-center luminescence dynamics has been investigated under low excitation density in Cd0.6Mn0.4Te/Cd0.5Mg0.5Te superlattices and Cd0.5Mn0.5Te thin film both experimentally and by Monte-Carlo simulation. A model developed earlier by the author for bulk Cd1−xMnxTe is applied to the superlattices with quantum wells of different width. It was demonstrated that under low temperature hopping-assisted quenching has been neglected in distinct of the significant quenching which was previously stated in bulk samples. In the superlattices with narrower quantum wells a homogeneous broadening is stated to be remarkably enhanced. To make clear an origin of the enhancement more experimental data are required. Noticeable non-exponential decay in Mn2+-luminescence transients is explained by stochasticity in decay rates, which are found to be completely correlated with the ions' energies. Partial correlation is stated to take place for the Mn2+-ions' energies of 4T1- and 4T2-states participating in the excitation process. The correlation loss is claimed to be originated from symmetry breakdown. Validity of the model applied to the superlattices is proved by good agreement of the experimental data and calculations under temperature rise from 13 to 77K.

Electron g-factor in GaAs/AlGaAs quantum wells of different width and barrier Al concentrations

Using electrically detected electron spin resonance technique, three components of electron g-factor tensor are studied in a number of asymmetrically doped GaAs/AlGaAs quantum wells grown in the [001] direction. The investigated quantum wells are characterized by different well widths and barrier Al concentrations. The dependence of gzz component along the growth direction on the heterostructure band gap was shown to be universal. The band gap was measured with the aid of photoluminescence. The two-fold in-plane g-factor anisotropy with the [110] and [11̄0] principal axes turned out to vanish in narrow quantum wells. Three nonzero components of â tensor of the linear in the magnetic field corrections to the g-factor are measured for a number of wells of different parameters. The â tensor components are strongly dependent on the well width.

Analytical model for threshold‐base current of a transistor laser with multiple quantum wells in the base

The authors investigate in this study the effect of incorporating multiple quantum wells (QWs) in the base of a heterojunction bipolar transistor laser on its performance. The expression for the terminal current is obtained by solving continuity equation in the base relating the bulk carrier density with the two-dimensional (2D) carrier density via virtual states. The gain in the QW is obtained by considering strain, 2D density-of-states, polarisation dependent momentum matrix element, Fermi statistics and Lorentzian broadening. The authors have taken as the basis for comparison the calculated as well as the experimental threshold-base current of 21.5 mA for a 16 nm wide InGaAs QW in GaAs base placed at 59 nm away from the emitter junction. Calculated value of 7.06 mA for three 16 nm wide QWs placed at 29, 59 and 79 nms in the base indicate substantial reduction in threshold-base current. Calculations are also made for three QWs of different widths having variable barrier widths. Reduction of threshold-base current by different amounts is observed in all cases.

The effect of barrier width in coupled asymmetric double quantum well structure on passive mode-locking region of existence

Semiconductor two-sectional laser diodes with active region, consisting of two InGaAs quantum wells of different width separated with GaAs barrier are investigated. At barrier thickness of 2 nm quantum wells are coupled and diagonal optical transition originates between them. Peak in absorption spectra of lasing structure related to this diagonal transition is observed. An additional region of passive mode-locking due to this absorption peak takes place at low reverse biases on absorber section.

Optical detection of an electric field in a GaAs/AlGaAs n-i-n heterostructure with double quantum wells

Processes occurring when a static transverse electric field is applied to a GaAs/AlGaAs n-i-n heterostructure with single quantum wells and asymmetric tunnel-coupled double quantum wells have been investigated by optical methods. The difference between the energies of exciton transitions for quantum wells of different widths makes it possible to attribute the observed photoluminescence peaks to particular pairs of wells or particular single quantum wells. The local electric field for each quantum well has been determined in terms of the Stark shift and splitting of exciton lines in a wide range of external voltage. A qualitative model has been proposed to explain the nonmonotonic distribution of the electric field over the depth of the heterostructure.

Efficiency droop in high‐Al‐content AlGaN/AlGaN quantum wells

AbstractExcitation power density dependence of photoluminescence (PL) efficiency is studied in the temperature range from 8 to 300 K for AlGaN multiple quantum well structures containing quantum wells of different width. PL efficiency droop is observed and dependence of the droop effect on the well width and lattice temperature is discussed. Photoluminescence band shape reveals strong carrier heating. It is shown that the heating effect has a substantial influence on the photoluminescence efficiency droop, especially at low lattice temperatures. Fitting of the temperature dependence of the PL band width with that obtained by the Monte Carlo simulation of carrier (exciton) hopping via localized states shows that (i) the potential fluctuations can be fairly accurately described using a double‐scaled potential profile model, (ii) the localization becomes stronger in narrower quantum wells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Conversion of bound excitons to free excitons by surface acoustic waves

We present detailed investigations of the surface acoustic wave-mediated dissociation of bound and free excitons in a GaAs multiquantum well. The energetic position of the photoluminescence spectrum can be assigned to the single quantum wells of different width and distance from the sample surface. The effect of the piezoelectric field on the efficiency of excition dissociation in the different quantum wells is characterized. For certain experimental conditions bound excitons are converted to free excitons by the surface acoustic wave and possible explanations are described.

Nonlinear magnetogyrotropic photogalvanic effect

We report on the observation of magnetic-field-induced photocurrent in HgTe/HgCdTe quantum wells of different widths. Both the intrasubband and interband absorption of infrared/terahertz radiation in the heterostructures is shown to cause a dc electric current in the presence of an in-plane magnetic field. The photocurrent behavior upon variation in the radiation polarization, magnetic-field strength, and temperature is studied. At a moderate magnetic field the current exhibits a linear field dependence. At high magnetic fields, however, it becomes nonlinear and is dominated by a cubic in magnetic-field contribution. The latter effect is observed in quantum wells with the inverted band structure only. The experimental results are analyzed in terms of the phenomenological theory and microscopic models of magnetogyrotropic photogalvanic effect based on asymmetry of optical transitions and/or asymmetric relaxation of carriers in the momentum space. The effect is shown to be related to the gyrotropic properties of the structures. The developed theory of magnetogyrotropic photocurrent describes well all experimental results. It is shown that both intrasubband and interband optical transitions may lead to spin-related as well as to spin-independent magnetic-field-induced photocurrents.

EFFECTS OF IONIZED IMPURITIES ON BINDING AND RECOMBINATION OF POSITIVE AND NEGATIVE QUASI-TWO-DIMENSIONAL MAGNETO-TRIONS

Binding energies of negative and positive trions in high magnetic fields are compared. Simultaneous inclusion of several Landau levels and quantum well subbands in exact numerical diagonalization allowed quantitative description of the coupling between in-plane dynamics (governed by interplay of cyclotron quantization and Coulomb interactions) and single-particle excitations in the normal direction. Symmetric and asymmetric GaAs quantum wells of different widths were considered, as well as the effect of a possible binding of trions by sparse ionized impurities nearby the quantum well.

Quantum well effect in bulk PbI2 crystals revealed by the anisotropy of photoluminescence and Raman spectra

On subjecting a bulk 2H-PbI2 crystal to vacuum annealing at 500 K followed by a sudden cooling at liquid nitrogentemperature stacking faults are generated that separate distinct layers of nanometricthickness in which different numbers of I–Pb–I atomic layers are bundled together. Suchstructures, containing two, three, four, five etc I–Pb–I atomic layers, behave as quantumwells of different widths. The signature of such a transformation is given by a shift towardshigher energies of the fundamental absorption edge, which is experimentally revealed byspecific anisotropies in the photoluminescence and Raman spectra. The quantumconfining effect is made visible by specific variations of a wide extra-excitonicband (G) at 2.06 eV that originates in the radiative recombination of carriers(electrons and holes), trapped on the surface defects. The excitation spectrum ofthe G band, with p polarized exciting light, reveals a fine structure comprisedof narrow bands at 2.75, 2.64, 2.59 and 2.56 eV, which are associated with thePbI2 quantum wells formed from two, three, four and five I–Pb–I atomic layers of0.7 nm thickness. Regardless of the polarization state of the laser excitinglight of 514.5 nm (2.41 eV), which is close to the band gap energy ofPbI2 (2.52 eV), the Raman scattering on bulk as-grownPbI2 crystals has the character of a resonant process. For p polarizedexciting light, the Raman scattering process on vacuum annealedPbI2 becomes non-resonant. This originates from the quantum well structures generated insidethe crystal, whose band gap energies are higher than the energy of the exciting light.

Optical study of a-plane InGaN/GaN multiple quantum wells with different well widths grown by metal-organic chemical vapor deposition

a -plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm grown on r-plane sapphire by metal-organic chemical vapor deposition were investigated. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL (TRPL) study shows that the sample with 3-nm-thick wells has the best optical property with a fastest exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify that the more uniform and stronger luminescence intensity distribution are observed for the samples of thinner quantum wells. In addition, more effective capturing of excitons due to larger localization energy Eloc and shorter radiative lifetime of localized excitons are observed in thinner well width samples in the temperature dependent TRPL.